Check valve substitution

Figure 2.26 Hydrocarbon fire from a flawed review of a check valve substitution.

Figure 2.27 Check valve substitution on 12 inch flammable piping.

In some cases, power-operated block valves may be substituted for the classic check valve. However, these valves depend on an outside motive... 

Where process liquid backflowto a ruptured heatertube is likely, a means to block this flow should be provided. A simple method is to install reliable check valves on the outlet lines. Remotely operated block valves on the outlet side can be substituted where process conditions might render check valves ineffective, e.g., coking, or where higher reliability or more positive shutoff is desired. Where it is possible to block both inlet and outlet lines on a heater. 

Maintenance located a 12 in. 300 psi RF check valve that was used on firewater pumps, but not on hydrocarbons. This substitute valve was manufactured to a Japanese flange to flange standard dimensioa The Japanese standard face to face dimensions are more compact than the ANSI standard so the storeroom stocked this valve with a connecting short spool piece of 12 in. piping (see Fig. 2.27 to imderstand the dimension). 

For the standby pump start test, which is an important test to ensure the pumps transfer without large pressure swings, a check should be made to see if the relief valves lift or the pressure falls to a pressure one half the difference between the standby pump start pressure and the compressor trip pressure. The transient pressure is best measured with a multipen chart recorder. The chart speed must be high enough to fully display the pressure variation. While not as good as the chart recorder, a simple shop-made test setup can be substituted. A spare switch is temporarily connected to the same location as the other switches. This switch is calibrated to close at the threshold acceptance pressure. The contacts are wired to a test light through a seal relay. The purpose of the relay is to maintain the cir-... 

The minimum flow often corresponds to maximum pressure drop and vice versa and these actual pressure drops are determined by system hydraulics (as discussed earlier). Also note that there are Cy corrections for viscous flow for cases where the valve Reynolds numbers are less than 5000. Finally, note that the valve characteristic type and rangeability should be checked, as detailed earlier. Although sizing equations in this sections use Fisher nomenclature, they are equivalent to ANSI/ISA versions if fI (where L signifies liquid) is substituted for Km-... 

---